In the 21st century, people are developing smart materials and smart sensors. The smart sensors, made of smart materials, provide in association with other components like actuators and control systems, the functional capability to react to internal and external environments and achieve adaptability. Examples include, but are not limited to, life science research involving the study of interaction between biological and other molecules, in-vitro diagnostics, food safety whereby bacteria and toxins are detected without the need for time-consuming growth experiments, fresh water control involving the detection of heavy metal ions in fresh water or terror-related compounds, such as, ricin in fresh water supplies and gas detection, detection of explosives, chemical warfare agents, narcotics and the like.
The realization that many molecular phenomena result in mechanical responses at the nanoscale level promises to bring about a revolution in the field of chemical, physical, and biological sensor development. Exploiting nanoscale mechanics for molecular recognition is a paradigm shift in sensor technology. In a quest for smaller, faster, better, smarter sensors, the micro-electromechanical systems (MEMS) have been scaled to the submicron range, leading to the new category of nanoelectromechanical systems (NEMS). The nanomechanical sensor opens additional potential for low-power high-frequency devices for mobile communications, or bio-sensors for the detection of single molecules by sensitized NEMS surfaces.
For example, the resonance frequency of a cantilever beam varies sensitively as a function of molecular adsorption. In addition, when the adsorption is confined to one side of the cantilever, the cantilever undergoes deflection due to adsorption-induced variation in surface free energy. Chemical selectivity can be achieved by coating the cantilevers with selective molecules. It is postulated that the cantilever bending depends on the changes in surface free energy while the resonance frequency variation is entirely due to mass loading (both specific and nonspecific adsorption). The minimum detectable adsorbed mass on a cantilever sensor can be increased by orders of magnitude by changing the dimensions of the device; smaller and thicker cantilevers offer higher resonance frequency and therefore better mass detection sensitivity.
Thus, a fast developing area of smart sensors is evolving, and an advanced combination of materials, sensors, actuators, control and processing are blended suitably to achieve devices with a huge potential for applications in many different fields.
Highly sensitive and selective detection of chemicals and biomolecules using integrated micro- and nanodevices have attracted much attention due to the importance in national security, clinical diagnostics, and industrial processes. Among these devices, nanomechanical cantilever sensors can detect surface stress or resonant frequency of a thin solid device induced by subtle molecular interactions. These cantilever nanomechanical sensors are robust and reliable in the detection and can be mass-produced using microfabrication techniques. Furthermore, their label-free and real-time detection abilities can be combined with microfluidics and microelectronics to create portable lab-on-chip systems. After the specific receptors are immobilized on one side of the cantilever, the interaction with certain molecules changes the surface stresses and leads to the bending of the cantilever.
Alternatively, the cantilever sensor can detect some physical changes such as temperature, relative humidity, viscosity, magnetic field, radiation and light intensity. These sensors can work in a dynamic mode in which the resonant frequency is monitored to reflect the change in the effective mass induced by molecule adsorption. Due to their small size and mass, nanomechanical cantilever sensors are sensitive to the mass changes induced by molecular scale recognition or adsorption, and have been used to detect nucleic acids, gaseous molecules, proteins, explosives, and other substances. The bending and the vibration of the cantilever can be measured using different approaches, including piezoresistive and piezoelectric method, optical interferometry and laser reflection methods. The laser reflection method is frequently utilized wherein a laser beam is used to illuminate the free end of a one-end fixed cantilever and changes in position are detected by a position sensitive detector.
In spite of the many advantages of the nanomechanical cantilever sensor, there are limitations and disadvantages of nanomechanical cantilever sensors regarding their adaptability and reconfigurablility. The first limitation is that for a certain magnitude of bending, it is necessary to determine whether it is due to the interaction with target species or not. This occurs because the specificity and selectivity of nanomechanical sensors comes only from immobilized molecules or receptors. The mechanical deformations of the cantilevers do not directly relate to molecular structure or property, even if the peak shape (ascending side and descending side) contains some specific molecular information. Although spectroscopic techniques (infrared and visible spectrometry, mass spectrometry, and nuclear magnetic resonance) could provide detailed information for molecular identification, adding scanning wavelength spectroscopic accessories to portable sensors increases their size, complexity, and expense, while reducing adaptability of the devices. Because one of the most important driving forces of creating a small sensor is to achieve wide and easy deployment as the result of compact size, simple structure and low price, combining the device with one spectroscopic method is thus not preferred.
Secondly, for most of the biomolecular detection applications, cantilever sensors have to take an extra step (e.g., heat treatment or stringent washing) to remove attached molecular species for the next detection. In other words, the sensor needs to recognize and detect molecules with adjustable sensitivity over a large concentration range call for an adaptive and reconfigurable sensing strategy.
Another limitation of nanomechanical cantilever sensors is the structural configuration wherein the sensor must have rectangular cross sections with thickness at sub-micrometer scale, and require that the top surface and the bottom surface be made of different material or be chemically modified. Usually a gold thin film is deposited on one side of the silicon substrate to reflect a laser beam and immobilize receptors. Such asymmetric bimorph thin film structures are sensitive to temperature change. The temperature must be controlled precisely using a thermoelectrically stabilized cell and stabilized for hours before detection, otherwise the specific binding signal cannot be differentiated from thermal noise.
Another limitation of the nanomechanical cantilever sensor is that during laser deflection detection, accurate bending measurements depend on the position of laser beam shining on the cantilever. However, 5-20% misplacement is normal because the size of the laser spot is larger than the actual width of the cantilever. A slight positional change of the laser spot on the cantilever changes the detection sensitivity. Therefore, an arbitrary bending of the sensor cannot be determined if the laser illumination position is changed due to a system perturbation. Such uncertainty makes it difficult to cross-compare the bending results.
It is important to note most surface modification based sensing devices, such as the nanomechanical cantilever sensors, do not possess the desired adaptability, reconfigurability and identification ability. These sensors have narrow detection or operation ranges, cannot detect target species at ultra low concentrations, and will be saturated at ultra high concentrations. Most importantly, all the surface modified sensors have to depend on either highly selective molecules or a large number of sensors in an array to determine the nature of molecules, making these sensors unsuitable for highly specific detection of species in uncontrolled field environments.
To place each sensing element in specific locations various techniques have been explored, but the deposition of sensing elements on contact electrodes with good registry is a low yield process and usually requires sophisticated instruments for simultaneous observation and electrode formation. It turns out that the manipulation of different sensing units is hard and time-consuming. The fabrication costs are prohibitively high for daily manufacture. When compared with instruments that are used in traditional chemical and biological analysis, micro- or nanoscale sensing devices are sensitive, integratable, relatively inexpensive, easy to be deployed and fast in response. However, unless a significant advancement in mechanism is realized, such devices are continuously challenged by the miniaturization of traditional instruments such as micro-mass spectrometers, optical sensors, and on-chip microanalysis systems.
The nanomechanical sensors of the present invention solve many problems and overcome many limitations in the prior art.